Ordinarily, I'd agree with you. I might even say Heidenheim is the slightly stronger team. But for me here, it's about hunger. Does Heidenheim want promotion more than Bremen to avoid relegation?
I generally believe, and this is based on the English championship anyway, that if you don't qualify for automatic promotion, then your energy's all sapped by the time you reach playoffs. And I have been rooting and watching Bremen play (that final game of the season was absurd!). They will want to win this, both of them. I predict a high-scoring game completely different from the first leg.
I should have bet earlier too, I was expecting odds to go UP but by the time I made my pick, Bremen below 2/1.
In most cases (8 out of 11) in previous relegation playoffs, the team from the first league had the upper hand in the end. But I don't really think this is a matter of energy as you suggested. The team from the second league has just this huge disadvantage of having a team that was put together to play in the second league and not the first. This sounds logical but let me explain.
The TV money that clubs from the second league are getting is substantially less than the one first league clubs receive. Sponsors aren't willing to pay that much so in the end they can't pay high salaries. And on top of that they can only offer 'second class football'. So all in all teams from the second Bundesliga neither have the money nor the reputation to lure star-players into their squad. With these prerequisites it is no wonder that teams from the second league have a hard time to compete with the one coming from the first league. They have to approach the games really from a fighting point because individually they are always on the losing side.
That's why some people even want the relegation playoffs to be abolished again. They think it's unfair that a team (like Werder) can play a completely shitty season and still have the chance to save themselves in the end while the team from the second league doesn't get rewarded for their good season.